Method for manufacturing printed circuit boards

ABSTRACT

A method for manufacturing a PCB is related. The method includes providing a substrate with two opposite conductive layers and defining a through hole passing through the two conductive layers; forming a first electrically conductive metal layer on an inner surface of the substrate in the through hole using an electro-less plating process; forming a second electrically conductive metal layer on the first electrically conductive metal layer using an electro-plating process till the through hole being filled.

BACKGROUND

1. Technical Field

The present disclosure generally relates to printed circuit boards, and particularly, relates to a method for manufacturing printed circuit boards (PCBs).

2. Description of Related Art

As technology progresses, microphones, portable computers and other electronic products have achieved ever greater levels of miniaturization by the use of double-sided PCBs and multilayer PCBs having via-holes and buried-holes.

Usually, a through hole metalizing process is employed in a traditional method for manufacturing double-sided PCBs or multilayer PCBs to establish electrical connection between circuits of different layers. The through hole metalizing process generally includes a step of filling electrically conductive material into a through hole of a PCB substrate having a base and two copper layers formed on opposite surfaces of the base. As such, the conductive material electrically connects the copper layers to each other.

However, the thermal expansion index of the electrically conductive material may be different from that of the copper layers. As a result, because of heating and cooling of the PCB, interspaces are formed between the conductive material and the PCB substrate. Thus, in wet etching process, etchant may be lodged into the interspaces and corrode the PCB substrate.

What is needed, therefore, is a method for manufacturing a printed circuit board to overcome the above-described problem.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the different views.

FIG. 1 is a flow chart showing a method for manufacturing a PCB according to a first embodiment.

FIG. 2 is a cross-sectional view of a substrate defining a through hole according to the first embodiment.

FIG. 3 is similar to FIG. 2, but showing a protecting layer is applied on an outer surface of the substrate with the through hole exposed.

FIG. 4 is similar to FIG. 3, but showing a first electrically conductive metal layer formed on an inner surface of the substrate defined in the though hole.

FIG. 5 is similar to FIG. 4, but showing a second electrically conductive metal layer formed on the first electrically conductive metal layer and the through hole completely filled by the second electrically conductive metal layer.

FIG. 6 is similar to FIG. 5, but showing the protecting layer removed.

FIG. 7 is similar to FIG. 6, but showing two ends of the second electrically conductive metal layer smoothened.

FIG. 8 is a cross-sectional view of a substrate defining a blind hole according to a second embodiment.

FIG. 9 is similar to FIG. 8, but showing a first electrically conductive metal layer formed on an inner surface defined in the through hole and an outer surface of the substrate.

FIG. 10 is similar to FIG. 9, but showing a protecting layer applied on an outer surface of the substrate.

FIG. 11 is similar to FIG. 10, but showing a second electrically conductive metal layer formed in the blind hole and the blind hole is filled by the second electrically conductive metal layer.

FIG. 12 is similar to FIG. 11, but showing the protecting material attached on the first electrically conductive metal layer removed.

FIG. 13 is similar to FIG. 12, but showing a portion of the first electrically conductive metal layer formed on the outer surface removed.

DETAILED DESCRIPTION

Referring to FIGS. 1-7, a method for manufacturing a PCB provided in a first embodiment includes the following steps.

In step 1, as shown in FIG. 2, a double-sided substrate 10 is provided. The substrate 10 includes a base 13, a first electrically conductive layer 11 and a second electrically conductive layer 12 formed on two opposite surfaces of the base 13. The first conductive layer 11 has a first outer surface 111, and the second conductive layer 12 has a second outer surface 121. The substrate 10 defines a through hole 101 therein passing through the first, second conductive layers 11, 12 and the base 13, and has an inner surface 1011 in the through hole 101.

The base 13 is an insulating resin layer, and the first conductive layer 11 and the second conductive layer 12 are made of copper. For purpose of manufacturing thin electrical traces, the thickness of the first conductive layer 11 and the second conductive layer 12 is less than or equal to 10 micrometers, and the diameter of the through hole 101 is in the range from 50 micrometers to 100 micrometers. Additionally, the base 13 can be an overlapping structure of a unit including an insulating resin layer and two conductive layers disposed on two opposite surfaces of the resin layer.

In step 2, as shown in FIG. 4, a first electrically conductive metal layer 20 is formed on the inner surface 1011 of the substrate 10 using an electro-less plating process.

In detail, as shown in FIG. 3, a protecting layer 31 is applied onto and fully cover each of the first and second outer surfaces 111, 121 of the substrate 10 except the through hole 101. The two protecting layers 31 are configured for protecting the first and second conductive layers 11, 13 from being plated. In the present embodiment, the two protecting layers 31 are dry photoresist films attached on the substrate 10 using a laminating process. Additionally, the two protecting layers 31 can be liquid photoresist layers applied using a coating process, or a known binder layer capable of being manually stripped from the substrate 10. Furthermore, for purpose of improving adhesion force between the protecting layers 31 and the first, second conductive layers 11, 12, prior to applying the protecting layers 31 onto the substrate 10, the first and second outer surfaces 111, 112 can be firstly cleared using a lye and secondly etched using a known etchant.

As shown in FIG. 4, the substrate 10 is subsequently plated using an electro-less plating process. In actual operation, before plating, the inner surface 1011 is cleared using a lye and etched to improve an adhesion force of metal layer contained in the electro-less plating process. Hence, a first electrically conductive metal layer 20 is formed on the inner surface 1011 of the substrate 10. The first electrically conductive metal layer 20 is made of copper and the thickness thereof is general less than 6 micrometers.

In step 3, as shown in FIG. 5, a second electrically conductive metal layer 40 is directly formed on the first electrically conductive metal layer 20 using an electro-plating process until the through hole 101 is entirely filled with the second electrically conductive metal layer 40. In actual operation, a portion of the obtained second electrically conductive metal layer 40 adjacent to the first electrically conductive metal layer 20 is thicker than the central portion thereof. In the present embodiment, the second electrically conductive metal layer 40 defines a first end 41 and an opposite second end 42. Both the first and second ends 41, 42 are arc-shaped in cross-section, and respectively extend beyond the first and second outer surfaces 111, 112.

As shown in FIG. 6, the protecting layers 31 on the substrate 10 are removed using a typical etching process. Thereafter, as shown in FIG. 7, the second electrically conductive metal layer 40 is smoothened using a roller 90 until both the first and second ends 41, 42 have a flat end surface. Additionally, when being a binder layer, the protecting layer 31 can be manually stripped, and the second electrically conductive metal layer 40 can be polished.

In the present embodiment, both the first and second electrically conductive metal layers 20, 40 are compact, and have approximate similar thermal expansion indexes to those of the two conductive layers 11, 12. Therefore, interspace is prevented from being formed between the two electrically conductive metal layers 20, 40 and the two conductive layers 11, 12, the inner surface 1011 is protected from corrosion in wet process.

Referring to FIGS. 8-12, a method for manufacturing a PCB is provided in a second embodiment, differing from the first embodiment is that the substrate 50 defining a blind hole 501 passing through only the first conductive layer 51 and the base 53, and exposing a portion of the second conductive layer 52. The substrate 50 defines an inner surface 5011 in the blind hole 501 and an outer surface 511 on the first conductive layer 51.

As shown in FIG. 9, the method includes a step of forming a first electrically conductive metal layer 60 on the substrate 50 using an electro-less plating process. The first electrically conductive metal layer 60 includes a first portion 61 formed on the outer surface 511 and a second portion 63 formed on the inner surface 5011 defined in the blind hole 501.

As shown in FIG. 10, the method includes a step of applying protecting layers 71 onto the substrate 50. The first portion 61 of the first metal layer 60 and the second conductive layer 52 are covered by the protecting layers 71 while the second portion 63 is exposed. The protecting layers 71 are dry photoresist films. Alternatively, the protecting layers 71 can be liquid photoresist layers or binder layers.

As showing in FIG. 11, the method includes a step of forming a second electrically conductive metal layer 80 on the second portion 63 using an electro-plating process until the blind hole 501 is filled with conductive metal layer 80.

Referring to FIG. 12, the method includes a step of removing a portion of the protecting layer 71 attached on the first portion 61 from the substrate 50 using a typical etching process or by manual stripping. Another portion of the protecting layer 71 attached on the second conductive layer 52 is kept for manufacturing electrical traces in subsequent process using a traditional exposing and developing method. Additionally, when the protecting layers 71 are binder layers, another portion of the protecting layers 71 attached on the second conductive layer 52 should be removed.

Referring to FIGS. 12˜13, the first portion 61 of the first electrically conductive metal layer 60 is removed. Therefore, the thickness of the first conductive layer 51 remains unchanged.

While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is not limited to the particular embodiments described and exemplified but is capable of considerable variation and modification without departure from the scope of the appended claims. 

1. A method for manufacturing a PCB, the method comprising: providing a substrate including a base and two conductive layers formed on opposite surfaces of the base, the substrate defining a through hole passing through the two conductive layers; forming a first electrically conductive metal layer on an inner surface of the substrate in the through hole using an electro-less plating process; forming a second electrically conductive metal layer on the first electrically conductive metal layer using an electro-plating process filling the through hole.
 2. The method as claimed in claim 1, further comprising applying a protecting layer onto outer surfaces of the substrate while exposing the through hole before forming the first electrically conductive metal layer.
 3. The method as claimed in claim 1, wherein the first electrically conductive metal layer includes a first portion formed on the outer surfaces of the substrate and a second portion formed on the inner surface, the method further comprising applying a protecting layer onto the first portion of the first electrically conductive metal layer while exposing the second portion thereof before forming the second electrically conductive metal layer.
 4. The method as claimed in claim 2, wherein the protecting layer is a dry photoresist film, liquid photoresist layer or binder layer.
 5. The method as claimed in claim 3, wherein the protecting layer is a dry photoresist film, liquid photoresist layer or binder layer.
 6. The method as claimed in claim 3, further comprising removing the protecting layer and the first portion of the first electrically conductive metal layer.
 7. The method as claimed in claim 1, further comprising smoothening opposite ends of the second electrically conductive metal layer, the opposite ends thereof protruding from the two opposite conductive layers.
 8. A method for manufacturing a PCB, comprising: providing a substrate, the substrate including a base and two conductive layers formed on opposite surfaces of the base and defining a blind hole passing through one of the two conductive layer and the base; forming a first electrically conductive metal layer on an inner surface of the substrate in the blind hole using an electro-less plating process; forming a second electrically conductive metal layer on the first electrically conductive metal layer using an electro-plating process till the through hole being filled.
 9. The method as claimed in claim 8, further comprising applying a protecting layer onto outer surfaces of the substrate while exposing the blind hole before forming the first electrically conductive metal layer.
 10. The method as claimed in claim 8, wherein the first electrically conductive metal layer includes a first portion formed on an outer surfaces of the substrate and a second portion formed on the inner surface, the method further comprising applying a protecting layer onto the first portion of the first electrically conductive metal layer while exposing the second portion thereof before forming the second electrically conductive metal layer.
 11. The method as claimed in claim 9, wherein the protecting layer is a dry photoresist film, liquid photoresist layer or binder layer.
 12. The method as claimed in claim 10, wherein the protecting layer is a dry photoresist film, liquid photoresist layer or binder layer.
 13. The method as claimed in claim 10, further comprising removing the protecting layer and the first portion of the first metal layer.
 14. The method as claimed in claim 8, further comprising smoothening opposite ends of the second electrically conductive metal layer, the opposite ends thereof protruding from the two opposite conductive layers. 